Process for making a fabricated article from polyolefin

ABSTRACT

The present disclosure describes a method for preparing a carbonized article comprising providing a fabricated polyolefin article; crosslinking the fabricated article with a boron-containing species (BCS); stabilizing the fabricated article by air oxidation; and carbonizing the fabricated article. The present disclosure further describes preparing a stabilized article.

BACKGROUND

Previously, carbonaceous articles, such as carbon fibers, have been produced primarily from polyacrylonitrile (PAN), pitch, or cellulose precursors. The process for making carbonaceous articles begins by forming a fabricated article, such as a fiber or a film, from the precursor. Precursors may be formed into fabricated articles using standard techniques for forming or molding polymers. The fabricated article is subsequently stabilized to allow the fabricated article to substantially retain shape during the subsequent heat-processing steps; without being limited by theory, such stabilization typically involves a combination of oxidation and heat and generally results in dehydrogenation, ring formation, oxidation and crosslinking of the precursor which defines the fabricated article. The stabilized fabricated article is then converted into a carbonaceous article by heating the stabilized fabricated article in an inert atmosphere. While the general steps for producing a carbonaceous article are the same for the variety of precursors, the details of those steps vary widely depending on the chemical makeup of the selected precursor.

Polyolefins have been investigated as an alternative precursor for carbonaceous articles, but a suitable and economically viable preparation process has proven elusive. Of particular interest is identifying an economical process for preparing carbonaceous articles from polyolefin precursors. For example, maximizing mass retention during the stabilization and carbonization steps is of interest.

STATEMENT OF INVENTION

The present disclosure describes a method for preparing a boron-treated carbonized article comprising providing a fabricated polyolefin article; crosslinking the fabricated article with a boron-containing species (BCS); stabilizing the fabricated article by air oxidation; and carbonizing the fabricated article. The present disclosure further describes preparing a stabilized article.

DETAILED DESCRIPTION

Unless otherwise indicated, numeric ranges, for instance “from 2 to 10,” are inclusive of the numbers defining the range (e.g., 2 and 10).

Unless otherwise indicated, ratios, percentages, parts, and the like are by weight.

Unless otherwise indicated, the crosslinkable functional group content for a polyolefin resin is characterized by the mol % crosslinkable functional groups, which is calculated as the number of mols of crosslinkable functional groups divided by the total number of mols of monomer units contained in the polyolefin.

Unless otherwise indicated, “monomer” refers to a molecule which can undergo polymerization, thereby contributing constitutional units to the essential structure of a macromolecule, for example, a polyolefin.

In one aspect, the present disclosure describes a process for producing a carbonaceous fabricated article from a polyolefin resin. Unless stated otherwise, any method or process steps described herein may be performed in any order. Polyolefins are a class of polymers produced from one or more olefin monomer. The polymers described herein may be formed from one or more types of monomers. Polyethylene is the preferred polyolefin resin, but other polyolefin resins may be substituted. For example, a polyolefin produced from ethylene, propylene, or other alpha-olefin (for instance, 1-butene, 1-hexene, 1-octene), or a combination thereof, is also suitable. The polyolefins described herein are typically provided in resin form, subdivided into pellets or granules of a convenient size for further melt or solution processing.

The polyolefin resins described herein are subjected to a crosslinking step. In one instance, the polyolefin resin is crosslinked using a Boron-containing species (BCS). In one instance, the polyolefin resins have been modified to include crosslinkable functional groups which are suitable for reacting in the presence of a BCS to crosslink the polyolefin resin. Any BCS suitable for initiating the formation of crosslinks in the polyolefin resin is suitable for use. Examples of suitable BCSs include borane, borate, borinic acid, boronic acid, boric acid, borinic or boronic ester, boroxine, aminoborane, borazine, borohydrides and derivatives and combinations thereof. In one instance, copolymers are suitable to provide a polyolefin resin having crosslinkable functional groups where one or more alpha-olefins have been copolymerized with another monomer containing a group suitable for serving as a crosslinkable functional group, for example, dienes, carbon monoxide, glycidyl methacrylate, acrylic acid, vinyl acetate, maleic anhydride, or vinyl trimethoxy silane (VTMS) are among the monomers suitable for being copolymerized with the alpha-olefin. Further, the polyolefin resin having crosslinkable functional groups may also be produced from a poly(alpha-olefin) which has been modified by grafting a functional group moiety onto the base polyolefin, wherein the functional group is selected based on its ability to subsequently enable crosslinking of the given polyolefin. For example, grafting of this type may be carried out by use of free radical initiators (such as peroxides) and vinyl monomers (such as VTMS, dienes, vinyl acetate, acrylic acid, methacrylic acid, acrylic and methacrylic esters such as glycidyl methacrylate and methacryloxypropyl trimethoxysilane, allyl amine, p-aminostyrene, dimethylaminoethyl methacrylate) or via azido-functionalized molecules (such as 4-[2-(trimethoxysilyl)ethyl]benzenesulfonyl azide). Polyolefin resins having crosslinkable functional groups may be produced from a polyolefin resin, or may be purchased commercially. Examples of commercially available polyolefin resins having crosslinkable functional groups include SI-LINK sold by The Dow Chemical Company, PRIMACOR sold by The Dow Chemical Company, EVAL resins sold by Kuraray, and LOTADER AX8840 sold by Arkema.

As described above, the polyolefin resin is processed to form a fabricated article. A fabricated article is an article which has been fabricated from the polyolefin resin. The fabricated article is formed using known polyolefin fabrication techniques, for example, melt or solution spinning to form fibers, film extrusion or film casting or a blown film process to form films, die extrusion or injection molding or compression molding to form more complex shapes, or solution casting. The fabrication technique is selected according to the desired geometry of the target carbonaceous article, and the desired physical properties of the same. For example, where the desired carbonaceous article is a carbon fiber, fiber spinning is a suitable fabrication technique. As another example, where the desired carbonaceous article is a carbon film, compression molding is a suitable fabrication technique.

As noted above, at least a portion of the polyolefin resin is crosslinked to yield a crosslinked fabricated article. In some embodiments, crosslinking is carried out via chemical crosslinking using a BCS. It is anticipated that the crosslinked fabricated article is crosslinked via multiple paths, for example, a portion of the crosslinked fabricated article may be crosslinked using a BCS and another portion of the crosslinked fabricated article may be crosslinked via another crosslinking method known in the art, for example, by use of irradiation or other non-boron containing mechanism. Thus, in some embodiments, the crosslinked fabricated article is a fabricated article which has been treated with one or more boron-containing chemical agents to crosslink the crosslinkable functional groups of the polyolefin resin having crosslinkable functional groups. Such chemical agent functions to initiate the formation of intramolecular chemical bonds between the crosslinkable functional groups or reacts with the crosslinkable functional groups to form intramolecular chemical bonds, as is known in the art. Chemical crosslinking causes the crosslinkable functional groups to react to form new bonds, forming linkages between the various polymer chains which define the polyolefin resin having crosslinkable functional groups. The chemical agent which effectuates the crosslinking is selected based on the type of crosslinkable functional group(s) included in the polyolefin resin; a diverse array of reactions are known which crosslink crosslinkable functional groups via intermolecular and intramolecular chemical bonds. A suitable chemical agent is selected which is known to crosslink the crosslinkable functional groups present in the fabricated article to produce the crosslinked fabricated article. For example, without limiting the present disclosure, if the crosslinkable functional group attached to the polyolefin is a vinyl group, suitable chemical agents include free radical initiators such as peroxides or azo-bis nitriles, for example, dicumyl peroxide, dibenzoyl peroxide, t-butyl peroctoate, azobisisobutyronitrile, and the like. If the crosslinkable functional group attached to the polyolefin is an acid, such as a carboxylic acid, or an anhydride, or an ester, or a glycidoxy group, a suitable chemical agent can be a compound containing at least two nucleophilic groups, including dinucleophiles such as diamines, diols, dithiols, for example ethylenediamine, hexamethylenediamine, butane diol, or hexanedithiol. Compounds containing more than two nucleophilic groups, for example glycerol, sorbitol, or hexamethylene tetramine can also be used. Mixed di- or higher-nucleophiles, which contain at least two different nucleophilic groups, for example ethanolamine can also be suitable chemical agents. If the crosslinkable functional group attached to the polyolefin is a mono-, di- or tri-alkoxy silyl group, water, and Lewis or Bronsted acid or base catalysts can be used as suitable chemical agents. For example, without limiting the present disclosure, Lewis or Bronsted acid or base catalysts include aryl sulfonic acids, sulfuric acid, hydroxides, zirconium alkoxides or tin reagents.

Crosslinking the fabricated article is generally preferred to ensure that the fabricated article retains its shape at the elevated temperatures required for the subsequent processing steps. Without crosslinking, polyolefin resins typically soften, melt or otherwise deform or breakdown at elevated temperatures. Crosslinking adds thermal stability to the fabricated article.

The crosslinked fabricated article is heated in an oxidizing environment to yield a stabilized fabricated article. In some embodiments, the temperature for stabilizing the crosslinked fabricated article is at least 120° C., preferably at least 190° C. In some embodiments, the temperature for stabilizing the crosslinked fabricated article is no more than 400° C., preferably no more than 300° C. In one instance, the crosslinked fabricated article is introduced to a heating chamber which is already at the desired temperature. In another instance, the fabricated article is introduced to a heating chamber at or near ambient temperature, which chamber is subsequently heated to the desired temperature. In some embodiments the heating rate is at least 1° C./minute. In other embodiments the heating rate is no more than 15° C./minute. In yet another instance, the chamber is heated step wise, for instance, the chamber is heated to a first temperature for a time, such as, 120° C. for one hour, then is raised to a second temperature for a time, such as 180° C. for one hour, and third is raised to a holding temperature, such as 250° C. for 10 hours. The stabilization process involves holding the crosslinked fabricated article at the given temperature for periods up to 100 hours depending on the dimensions of the fabricated article. The stabilization process yields a boron-treated stabilized fabricated article which is a precursor for a carbonaceous article. Without being limited by theory, the stabilization process oxidizes the crosslinked fabricated article and causes changes to the hydrocarbon structure that increases the crosslink density while decreasing the hydrogen/carbon ratio of the crosslinked fabricated article. In one instance, the stabilization process introduces boron to the hydrocarbon structure by treating the fabricated article with boron during the stabilization process.

Unexpectedly, it has been found that including a BCS in the oxidizing environment improves mass retention of the subsequently produced stabilized article. Unexpectedly, it has been found that including a BCS in the fabricated article during the stabilization step improves mass retention of the subsequently produced stabilized carbonaceous article. It has also been found that treating the crosslinked fabricated article with a boron-containing species improves form-retention of the subsequently produced carbonaceous article.

In another aspect, the present disclosure describes a boron-treated stabilized fabricated article which is formed from a polyolefin precursor (resin). In one instance, the boron-treated stabilized fabricated article is formed according to the process described herein.

In yet another aspect, a carbonaceous article and a process for making the same are provided. Carbonaceous articles are articles which are rich in carbon; carbon fibers, carbon sheets and carbon films are examples of carbonaceous articles. Carbonaceous articles have many applications, for example, carbon fibers are commonly used to reinforce composite materials, such as in carbon fiber reinforced epoxy composites, while carbon discs or pads are used for high performance braking systems.

The carbonaceous articles described herein are prepared by carbonizing the stabilized fabricated article by heat-treating the boron-treated stabilized fabricated articles in an inert environment. The inert environment is an environment surrounding the boron-treated stabilized fabricated article that shows little reactivity with carbon at elevated temperatures, preferably a high vacuum or an oxygen-depleted atmosphere, more preferably a nitrogen atmosphere or an argon atmosphere. It is understood that trace amounts of oxygen may be present in the inert atmosphere. In one instance, the temperature of the inert environment is at or above 600° C. Preferably, the temperature of the inert environment is at or above 800° C. In one instance, the temperature of the inert environment is no more than 3000° C. In one instance, the temperature is from 1400-2400° C. Temperatures at or near the upper end of that range will produce a graphite article, while temperatures at or near the lower end of the range will produce a carbon article.

In order to prevent bubbling or damage to the fabricated article during carbonization, it is preferred to heat the inert environment in a gradual or stepwise fashion. In one embodiment, the boron-treated stabilized fabricated article is introduced to a heating chamber containing an inert environment at or near ambient temperature, which chamber is subsequently heated over a period of time to achieve the desired final temperature. The heating schedule can also include one or more hold steps for a prescribed period at the final temperature or an intermediate temperature or a programmed cooling rate before the article is removed from the chamber.

In yet another embodiment, the chamber containing the inert environment is subdivided into multiple zones, each maintained at a desired temperature by an appropriate control device, and the boron-treated stabilized fabricated article is heated in a stepwise fashion by passage from one zone to the next via an appropriate transport mechanism, such as a motorized belt. In the instance where a boron-treated stabilized fabricated article is a fiber, this transport mechanism can be the application of a traction force to the fiber at the exit of the carbonization process while the tension in the stabilized fiber is controlled at the inlet. Some embodiments of the invention will now be described in detail in the following Examples.

In the Examples, overall mass yield is calculated as the product of oxidation mass yield and carbonization mass yield. PHR refers to parts per hundred resin (mass basis). MI refers to melt index which is a measure of melt flow rate. Wt % refers to parts per 100 total parts, mass basis. PE refers to polyethylene. BA refers to boric acid. Definitions of measured yields

Oxidation mass yield:

$Y_{O} = \frac{m_{OX}}{m_{PE}}$

Carbonization mass yield:

$Y_{C} = \frac{m_{CF}}{m_{OX}}$

Overall mass yield: Y_(M)=Y_(O)Y_(C)

Overall mass yield (carbonaceous mass per initial mass of PE):

$Y_{M,{PE}} = \frac{Y_{O}Y_{C}}{M_{\% {PE}}}$

Where m_(PE) is the initial mass of polyethylene; m_(OX) is the mass remaining after oxidation; m_(CF) is the mass remaining after carbonization; M_(% PE) is the mass % of polyethylene in the origin formed article.

Soxhlet extraction is a method for determining the gel content and swell ration of crosslinked ethylene plastics. As used herein, Soxhlet extraction is conducted according to ASTM Standard D2765-11 “Standard Test Methods for Determination of Gel Content and Swell Ratio of Crosslinked Ethylene Plastics.” In the method employed, a crosslinked fabricated article between 0.050-0.500 g is weighed and placed into a cellulose-based thimble which is then placed into a Soxhlet extraction apparatus with sufficient quantity of xylenes. Soxhlet extraction is then performed with refluxing xylenes for at least 12 hours. Following extraction, the thimbles are removed and the crosslinked fabricated article is dried in a vacuum oven at 80° C. for at least 12 hours and then weighed, thereby providing a Soxhlet-treated article. The gel content (%) is then calculated from the weight ratio (Soxhlet-treated article)/(crosslinked fabricated article).

Example 1

An ethylene/octene copolymer (density=0.941 g/cm³; MI=34 g/10 min, 190° C./2.16 kg) is reactive extruded with vinyl trimethoxysilane (VTMS) to form a VTMS-grafted ethylene/octene copolymer (MI=19 g/10 min, 190° C./2.16 kg; 1.4 wt % grafted silane content determined by ¹³C NMR) precursor resin. The VTMS-grafted precursor resin is melt spun to form fibers with the following properties: 1573 filaments, 1945.8 total denier, 2.25 gf/den, 12.17% elongation-to-break. Fiber tows are continuously treated in a vessel containing an isopropanol solution with 5 wt % of boric acid. Fiber residence time in the solution is 5 seconds. The treated fibers are allowed to dry cure for 3 days. The fibers are subsequently moisture cured at 80° C. (100% relative humidity) for 1-5 days, as reported in Table 1. Gel fraction is determined to be 42.9-55.2% by Soxhlet extraction. Complete results are reported in Table 2.

TABLE 1 Segment Moisture Curing Time (days) A 1 B 3 C 5

TABLE 2 Segment Gel Fraction (%) A 42.9 B 53.3 C 55.2

The boric acid crosslinked fibers are oxidized and carbonized using a Thermogravimetric Analysis (TGA) instrument using the conditions outlined in Table 3. The temperature ramp rates are maintained at 10° C./min for the oxidation and carbonization regimes. Table 4 reports the mass retained during air oxidation and final mass yield after both oxidation and carbonization treatments.

TABLE 3 Oxidation (air) Isothermal Carbonization (nitrogen) Hold, Starting Final Isothermal Temperature Temperature Temperature Segment Hold, Time (hr) (° C.) (° C.) (° C.) 1 5 270 270 800

TABLE 4 Mass retained during Segment oxidation (%) Mass yield (%) A1 44.24 17.11 B1 50.11 15.16 C1 49.26 14.11

Example 2

An ethylene/octene copolymer (density=0.941 g/cm³; MI=34 g/10 min, 190° C./2.16 kg) is reactive extruded with vinyl trimethoxysilane (VTMS) to form a VTMS-grafted ethylene/octene copolymer (MI=19 g/10 min, 190° C./2.16 kg; 1.4 wt % grafted silane content determined by ¹³C NMR) precursor resin. The VTMS-grafted precursor resin is melt spun to form fibers with the following properties: 1573 filaments, 1945.8 total denier, 2.25 gf/den, 12.17% elongation-to-break. The fiber tows are continuously treated in a vessel containing an isopropanol solution with 5 wt % of boric acid. The fiber residence time in the vessel is 5 minutes. The treated fibers are allowed to dry cure for 3 days. The fibers are subsequently moisture cured at 80° C. (100% relative humidity) for 1-5 days, as reported in Table 5. Gel fraction is determined to be 42.4-54.1% by Soxhlet extraction. Complete results are reported in Table 6.

TABLE 5 Segment Moisture Curing Time (days) D 1 E 3 F 5

TABLE 6 Segment Gel Fraction (%) D 42.4 E 50.8 F 54.1

The resulting boric acid crosslinked fibers are oxidized and carbonized using a Thermogravimetric Analysis (TGA) instrument using the conditions outlined in Table 7. Temperature ramp rates are maintained at 10° C./min for oxidation and carbonization regimes. Table 8 reports the mass retained during air oxidation and final mass yield after both oxidation and carbonization treatments.

TABLE 7 Oxidation (air) Isothermal Carbonization (nitrogen) Hold, Starting Final Isothermal Temperature Temperature Temperature Segment Hold, Time (hr) (° C.) (° C.) (° C.) 1 5 270 270 800

TABLE 8 Mass retained during Segment oxidation (%) Mass yield (%) D1 46.33 16.86 E1 48.32 16.77 F1 49.46 16.34

Example 3

Three segments (A, B, and C) prepared and crosslinked according to Example 1 are treated with a 15 wt % solution of boric acid in methanol for various times reported in Table 9. After the boric acid solution treatment, the fibers are dried overnight in air at ambient conditions. The dried, boric acid treated fibers next undergo thermal treatment (80° C.) overnight in a vacuum oven. Mass of the fiber prior to and after boric acid treatment and relative change in mass are reported in Table 10.

TABLE 9 Segment Boric acid treatment time (min) 1 5 2 30

TABLE 10 Mass of fiber Mass of fiber before BA after BA Mass change Segment treatment (g) treatment (g) (%) A1 0.2418 0.2877 18.98 B1 0.2476 0.3013 21.69 C1 0.2315 0.2732 18.01 A2 0.2600 0.3104 19.38 B2 0.2338 0.2854 22.07 C2 0.2356 0.2858 21.31

The thermally treated, boric acid treated crosslinked fibers are oxidized and carbonized using a Thermogravimetric Analysis (TGA) instrument using the conditions outlined in Table 11. Temperature ramp rates are maintained at 10° C./min for oxidation and carbonization regimes. Table 12 reports the mass retained during air oxidation and final mass yield after both oxidation and carbonization treatments.

TABLE 11 Oxidation (air) Isothermal Carbonization (nitrogen) Hold, Starting Final Isothermal Temperature Temperature Temperature Segment Hold, Time (hr) (° C.) (° C.) (° C.) A-C 5 270 270 800

TABLE 12 Mass retained during Overall Mass Yield Segment oxidation (%) Char yield (%) (%, per mass PE) A1 68.78 45.87 56.62 B1 67.71 45.44 58.03 C1 71.35 39.46 48.13 A2 68.11 43.81 54.34 B2 68.04 43.87 56.29 C2 68.92 43.96 55.86 

What is claimed is:
 1. A method for preparing a carbonized article comprising: (a) providing a polyolefin resin; (b) forming a fabricated article from the polyolefin resin; (c) crosslinking the fabricated article with a boron-containing species (BCS); (d) stabilizing the fabricated article by air oxidation; and (e) carbonizing the fabricated article.
 2. The method of claim 1, wherein the BCS comprises borane, borate, borinic acid, boronic acid, boric acid, borinic or boronic ester, boroxine, aminoborane, borazine, borohydrides and derivatives and combinations thereof.
 3. The method of claim 1, wherein the method further comprises treating the crosslinked fabricated article prepared in step (c) with a BCS.
 4. The method of claim 1, wherein step (d) comprises heating the crosslinked fabricated article at or above 120° C.
 5. The method of claim 1, wherein step (b) comprises converting said polyolefin resin to a fabricated article by fiber spinning, film extrusion casting, blown film processing, profile extrusion through a die, injection molding, solution casting or compression molding.
 6. A method for preparing a boron-treated carbonized article comprising: (a) providing a fabricated polyolefin article; (b) crosslinking the fabricated article with a boron-containing species (BCS); (c) stabilizing the fabricated article by air oxidation; and (d) carbonizing the fabricated article.
 7. The method of claim 6, wherein the BCS comprises borane, borate, borinic acid, boronic acid, boric acid, borinic or boronic ester, boroxine, aminoborane, borazine, borohydrides and derivatives and combinations thereof.
 8. The method of claim 6, wherein the method further comprises treating the crosslinked fabricated article prepared in step (b) with a BCS.
 9. The method of claim 6, wherein step (c) comprises heating the crosslinked fabricated article at or above 120° C.
 10. A method for preparing a stabilized article comprising: (a) providing a polyolefin fabricated article; (b) crosslinking the fabricated article with a boron-containing species (BCS); and (c) stabilizing the fabricated article by air oxidation. 